General illumination devices are typically rated in terms of their color reproduction. Color reproduction is typically measured using the Color Rendering Index (CRI Ra). CRI Ra is a modified average of the relative measurements of how the color rendition of an illumination system compares to that of a reference radiator when illuminating eight reference colors, i.e., it is a relative measure of the shift in surface color of an object when lit by a particular lamp. The CRI Ra equals 100 if the color coordinates of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the reference radiator.
Daylight has a high CRI (Ra of approximately 100), with incandescent bulbs also being relatively close (Ra greater than 95), and fluorescent lighting being less accurate (typical Ra of 70-80). Certain types of specialized lighting have very low CRI (e.g., mercury vapor or sodium lamps have Ra as low as about 40 or even lower). Sodium lights are used, e.g., to light highways—driver response time, however, significantly decreases with lower CRI Ra values (for any given brightness, legibility decreases with lower CRI Ra). See Commission Internationale de l'Eclairage. Method of Measuring and Specifying Colour Rendering Properties of Light Sources, CIE 13.3 (1995) for further information on CRI.
The color of visible light output by a light emitter, and/or the color of blended visible light output by a plurality of light emitters can be represented on either the 1931 CIE (Commission International de I'Eclairage) Chromaticity Diagram or the 1976 CIE, Chromaticity Diagram. Persons of skill in the art are familiar with these diagrams, and these diagrams are readily available (e.g., by searching “CIE Chromaticity Diagram” on the internet).
The CIE Chromaticity Diagrams map out the human color perception in terms of two CIE parameters x and y (in the case of the 1931 diagram) or u′ and v′ (in the case of the 1976 diagram). Each point (i.e., each “color point”) on the respective Diagrams corresponds to a particular hue. For a technical description of CIE chromaticity diagrams, see, for example, “Encyclopedia of Physical Science and Technology”, vol. 7, 230-231 (Robert A Meyers ed., 1987). The spectral colors are distributed around the boundary of the outlined space, which includes all of the hues perceived by the human eye. The boundary represents maximum saturation for the spectral colors.
The 1931 CIE Chromaticity Diagram can be used to define colors as weighted sums of different hues. The 1976 CIE Chromaticity Diagram is similar to the 1931 Diagram, except that similar distances on the 1976 Diagram represent similar perceived differences in color.
In the 1931 Diagram, deviation from a point on the Diagram (i.e., “color point” or hue) can be expressed either in terms of the x, y coordinates or, alternatively, in order to give an indication as to the extent of the perceived difference in color, in terms of MacAdam ellipses. For example, a locus of points defined as being ten MacAdam ellipses from a specified hue defined by a particular set of coordinates on the 1931 Diagram consists of hues that would each be perceived as differing from the specified hue to a common extent (and likewise for loci of points defined as being spaced from a particular hue by other quantities of MacAdam ellipses). A typical human eye is able to differentiate between hues that are spaced from each other by more than seven MacAdam ellipses (but is not able to differentiate between hues that are spaced from each other by seven or fewer MacAdam ellipses).
Since similar distances on the 1976 Diagram represent similar perceived differences in color, deviation from a point on the 1976 Diagram can be expressed in terms of the coordinates, u′ and v′, e.g., distance from the point=(Δu′2+Δv′2)1/2. This formula gives a value, in the scale of the u′ v′ coordinates, corresponding to the distance between points. The hues defined by a locus of points that are each a common distance from a specified color point consist of hues that would each be perceived as differing from the specified hue to a common extent.
A series of points that is commonly represented on the CIE Diagrams is referred to as the blackbody locus. The chromaticity coordinates (i.e., color points) that lie along the blackbody locus obey Planck's equation: E(λ)=Aλ−5/(e(B/T)−1), where E is the emission intensity, λ is the emission wavelength, T is the color temperature of the blackbody and A and B are constants. The 1976 CIE; Diagram includes temperature listings along the blackbody locus. These temperature listings show the color path of a blackbody radiator that is caused to increase to such temperatures. As a heated object becomes incandescent, it first glows reddish, then yellowish, then white, and finally blueish. This occurs because the wavelength associated with the peak radiation of the blackbody radiator becomes progressively shorter with increased temperature, consistent with the Wien Displacement Law. Illuminants that produce light that is on or near the blackbody locus can thus be described in terms of their color temperature.
Light emitting diode lamps have been demonstrated to be able to produce white light with component efficacy>150 L/W and are anticipated to be the predominant lighting devices within the next decade. See e.g., Narukawa, Narita, Sakamoto, Deguchi, Yamada, Mukai: “Ultra-High Efficiency White Light Emitting Diodes” Jpn. J. Appl. Phys. 32 (1993) L9 Vol. 45, No. 41, 2006, pp. L1084-L10-86; and on the World Wide Web nichia.com/about_nichia/2006/2006—122001.html.
Many systems are based primarily on LEDs which combine blue emitters+YAG:Ce or BOSE phosphors or Red, Green and Blue InGaN/AlInGaP LEDs; or UV LED excited RGB phosphors. These methods have good efficacy but only medium CRI or very good CRI and low efficacy. The efficacy and CRI tradeoff in LEDs is also an issue in the lighting industry with regard to fluorescent illumination. See Zukauskas A., Shur M. S., Cacka R. “Introduction to Solid-State Lighting” 2002, ISBN 0-471-215574-0, section 6.1.1 page 118.
CRI Ra is the most commonly used metric for measuring color quality today. This CIE standard method (see, e.g., Commission Internationale de l'Eclairage, Method of Measuring and Specifying Colour Rendering Properties of Light Sources, CIE 13.3 (1995)) compares the rendered colors of 8 reference color swatches illuminated by the test illumination to the rendered color of the same swatches illuminated by reference light. Illumination with a CRI Ra of less than 50 is very poor and only used in applications where there is no alternative for economic issues. Lights with a CRI Ra between 70 and 80 have application for general illumination where the colors of objects are not important. For some general interior illumination, a CRI Ra of at least 80 is acceptable.
The whiteness of the emission from a lighting device is somewhat subjective. In terms of illumination, it is generally defined as to its proximity to the planckian blackbody locus (“BBL”). Schubert, in his book Light-Emitting Diodes, second edition, on page 325 states, “the pleasantness and quality of white illumination decreases rapidly if the chromaticity point of the illumination source deviates from the planckian locus by a distance of greater than 0.01 in the x,y chromaticity system. This corresponds to the distance of about 4 MacAdam ellipses, a standard employed by the lighting industry. See Duggal A. R. “Organic electroluminescent devices for solid-state lighting” in Organic Electroluminescence edited by Z. H. Kafafi (Taylor and Francis Group, Boca Raton, Fla., 2005). Note the 0.01-rule-of-thumb is a necessary but not a sufficient condition for high quality illumination sources.” A lighting device which has color coordinates that are within 4 MacAdam step ellipses of the planckian locus and which has a CRI Ra>80 is generally acceptable as a white light for illumination purposes. A lighting device which has color coordinates within 7 MacAdam ellipses of the planckian locus and which has a CRI Ra>70 is used as the minimum standard for many other white lighting devices including CFL and SSL (solid state lighting) lighting devices. (see DOE-Energy Star Program requirements for SSL Luminaires, 2006). A light with color coordinates within 4 MacAdam step ellipses of the planckian locus and a CRI Ra>85 is more suitable for general illumination purposes. CRI Ra>90 is preferable and provides greater color quality.
Some of the most commonly used LEDs in solid state lighting are phosphor excited LEDs. In many instances, a yellow phosphor (typically YAG:Ce or BOSE) is coated on a blue InGaN LED die. The resultant mix of yellow phosphor emitted light and some leaking blue light combines to produce a white light. This method typically produces light>5000K CCT and typically has a CRI Ra of between ˜70 and 80. For warm white colors, an orange phosphor or a mix of red and yellow phosphor can be used.
Light made from combinations of standard “pure colors,” red, green and blue, exhibit poor efficacy due primarily to the poor quantum efficiency of green LEDs. R+G+B lights also suffer from lower CRI Ra, in part due to the narrow full width at half maximum (FWHM) values of the green and red LEDs. Pure color LEDs (i.e., saturated LEDs) usually have a FWHM value in the range of from about 15 nm to about 30 nm.
UV based LEDs combined with red, green and blue phosphors offer quite good CRI Ra, similar to fluorescent lighting. Due to increased Stokes losses, however, they also have lower efficacies.
The highest efficiency LEDs today are blue LEDs made from InGaN. Commercially available devices have external quantum efficiency (EQE) as great as 60%. The highest efficiency phosphors suitable for LEDs today are YAG:Ce and BOSE phosphor with a peak emission around 555 nm. YAG:Ce has a quantum efficiency of >90% and is an extremely robust and well tested phosphor. Using this approach, almost any color along the tie line between the hue of the LED and the hue of the phosphor (e.g., FIG. 1 shows a tie line between a blue LED (i.e., an LED that emits blue light) that has a peak wavelength of about 455 nm and a yellow phosphor that has a dominant wavelength of about 569 nm).
In many lighting devices, the portion of the lumens of blue light is greater than approximately 3% and less than approximately 7%, and the combined emitted light appears white and falls within the generally acceptable color boundaries of light suitable for illumination. Efficacy as high as 150 L/W has been reported for LEDs made in this area, but commercially available lamps generally have CRI Ra in the range of from 70 to 80.
White LED lamps made with this method typically have a CRI Ra of between 70 and 80, the primary omission from the spectrum being red color components and, to some extent, cyan.
Red AlInGaP LEDs have very high internal quantum efficiency, but due to the large refractive index mismatch between AlInGaP and suitable encapsulant materials, a lot of light is lost due to total internal reflection (TIR). Regardless, red and orange packaged LEDs are commercially available with efficacies higher than 60 L/W.
Additional information on LEDs for general illumination, shortcomings and potential solutions may be found in “Light Emitting Diodes (LEDs) for General Illumination” OIDA, edited by Tsao J. Y, Sandia National Laboratories, 2002.
U.S. Pat. No. 7,095,056 (Vitta '056) discloses a white light emitting device and method that generate light by combining light produced by a white light source (i.e., light which is perceived as white) with light produced by at least one supplemental light emitting diode (LED). In one aspect, Vitta '056 provides a device which comprises a light source which emits light which would be perceived as white, a first supplemental light emitting diode (LED) that produces cyan light, and a second supplemental LED that produces red light, wherein the light emitted from the device comprises a combination of the light produced by the white light source, the first supplemental LED, and the second supplemental LED. While the arrangement disclosed in Vitta '056 allows the CCT to be changed, the CRI and the usefulness of the device reduces significantly at lower color temperatures, making this arrangement generally undesirable for indoor general illumination.
One technique for providing high efficiency and high color rendering is described in U.S. Pat. No. 7,213,940. The '940 patent describes combining non-white light with red/red-orange light to provide high color rendering and high efficiency. The teachings of the '940 patent are implemented in the TrueWhite technology incorporated in the LR6 6″ recessed downlight, and the LR24 2′×2′ architectural lay-in fixture from Cree, Inc. of Durham, N.C. The LR6 and the LR24 use phosphor converted LEDs that provide a blue LED and a YAG phosphor to provide blue-shifted-yellow (“BSY”) light that is combined with light from red LEDs to provide white light with a CCT of 2700K or 3500K and a CRI of greater than 90. FIG. 2 illustrates how a non-saturated non-white phosphor converted LED and a red/orange LED can be combined to provide white light.
The expression “phosphor converted” is used herein to mean a light emitter that includes an excitation emitter (e.g., a light emitting diode) and at least one phosphor, in which the excitation emitter generates light of a first wavelength, at least a portion of which is absorbed by the phosphor and re-emitted by the phosphor (in at least one different wavelength, typically in a range of wavelengths), whereby light of the first wavelength mixes with light re-emitted by the phosphor.
FIG. 3 is a schematic diagram of the LR6 and LR24 fixtures. As seen in FIG. 3, the LR6 and LR24 each have three strings of LEDs. Two of the strings include BSY LEDs and a third string includes red LEDs. The BSY LEDs are selected from two or more bins to provide a combined color point that is approximately opposite the BBL from the dominant wavelength of the red LEDs. The current through the red LEDs is then adjusted to pull the color point of the BSY LEDs to the BBL. Details on the operation of the LR6 and LR24 are found in:
U.S. patent application Ser. No. 11/755,153, filed May 30, 2007 (now U.S. Patent Publication No. 2007/0279903), the entirety of which is hereby incorporated by reference as if set forth in its entirety;
U.S. patent application Ser. No. 11/859,048, filed Sep. 21, 2007 (now U.S. Patent Publication No. 2008/0084701), the entirety of which is hereby incorporated by reference as if set forth in its entirety;
U.S. Pat. No. 7,213,940 , issued on May 8, 2007, the entirety of which is hereby incorporated by reference as if set forth in its entirety;
U.S. Patent Application No. 60/868,134, filed on Dec. 1, 2006, entitled “LIGHTING DEVICE AND LIGHTING METHOD” , the entirety of which is hereby incorporated by reference as if set forth in its entirety;
U.S. patent application Ser. No. 11/948,021, filed on Nov. 30, 2007 (now U.S. Patent Publication No. 2008/0130285), the entirety of which is hereby incorporated by reference as if set forth in its entirety;
U.S. patent application Ser. No. 12/475,850, filed on Jun. 1, 2009 (now U.S. Patent Publication No. 2009-0296384), the entirety of which is hereby incorporated by reference as if set forth in its entirety;
U.S. patent application Ser. No. 11/877,038, filed Oct. 23, 2007 (now U.S. Patent Publication No. 2008/0106907), the entirety of which is hereby incorporated by reference as if set forth in its entirety;
U.S. patent application Ser. No. 12/248,220, filed on Oct. 9, 2008 (now U.S. Patent Publication No. 2009/0184616), the entirety of which is hereby incorporated by reference as if set forth in its entirety;
U.S. patent application Ser. No. 11/947,392, filed on Nov. 29, 2007 (now U.S. Patent Publication No. 2008/0130298), the entirety of which is hereby incorporated by reference as if set forth in its entirety;
U.S. patent application Ser. No. 12/117,280, filed May 8, 2008 (now U.S. Patent Publication No. 2008/0309255), the entirety of which is hereby incorporated by reference as if set forth in its entirety;
U.S. patent application Ser. No. 12/257,804, filed on Oct. 24, 2008 (now U.S. Patent Publication No. 2009/0160363), the entirety of which is hereby incorporated by reference as if set forth in its entirety;
U.S. patent application Ser. No. 12/328,144, filed Dec. 4, 2008 (now U.S. Patent Publication No. 2009/0184666), the entirety of which is hereby incorporated by reference as if set forth in its entirety;
U.S. patent application Ser. No. 12/116,346, filed May 7, 2008 (now U.S. Patent Publication No. 2008/0278950), the entirety of which is hereby incorporated by reference as if set forth in its entirety;
U.S. patent application Ser. No. 12/116,348, filed on May 7, 2008 (now U.S. Patent Publication No. 2008/0278957), the entirety of which is hereby incorporated by reference as if set forth in its entirety; and
U.S. patent application Ser. No. 12/328,115, filed on Dec. 4, 2008 (now U.S. Patent Publication No. 2009-0184662), the entirety of which is hereby incorporated by reference as if set forth in its entirety.
The LR6 and LR24 each provide a CRI of greater than 90. Phosphor converted BSY LEDs with increased brightness have become available, the wavelength of the underlying excitation blue LED of these brighter BSY LEDs being lower. With this decrease in blue LED wavelength, it may become more difficult to achieve the desired high CRI. To overcome this issue, the LR6-230V has been fabricated to include a longer wavelength supplemental blue LED that replaces one of the BSY LEDs as shown in FIG. 3 and as described in U.S. patent application Ser. No. 12/248,220, filed on Oct. 9, 2008 (now U.S. Patent Publication No. 2009/0184616), the entirety of which is hereby incorporated by reference as if set forth in its entirety. A schematic diagram of the LR6-230V is provided as FIG. 4.